Gas turbine engines are known to include a compressor for compressing air; a combustor for producing a hot gas by burning fuel in the presence of the compressed air produced by the compressor, and a turbine for expanding the hot gas to extract shaft power. Diffusion flames burning at or near stoichiometric conditions with flame temperatures exceeding 3,000° F. dominate the combustion process in many older gas turbine engines. Such combustion will produce a high level of oxides of nitrogen (NOx). Current emissions regulations have greatly reduced the allowable levels of NOx emissions. Lean premixed combustion has been developed to reduce the peak flame temperatures and to correspondingly reduce the production of NOx in gas turbine engines. In a premixed combustion process, fuel and air are premixed in a premixing section of the combustor. The fuel-air mixture is then introduced into a combustion chamber where it is burned.
The design of a gas turbine combustor is complicated by the necessity for the gas turbine engine to operate reliably with a low level of emissions at a variety of power levels. High power operation at high firing temperatures tends to increase the generation of oxides of nitrogen. Low power operation at lower combustion temperatures tends to increase the generation of carbon monoxide and unburned hydrocarbons due to incomplete combustion of the fuel. Under all operating conditions, it is important to ensure the stability of the flame to avoid unexpected flameout, damaging levels of acoustic vibration, and damaging flashback of the flame from the combustion chamber into the fuel premix section of the combustor. A relatively rich fuel/air mixture will improve the stability of the combustion process but will have an adverse affect on the level of emissions. A careful balance must be achieved among these various constraints in order to provide a reliable machine capable of satisfying very strict modern emissions regulations.
Staging is the delivery of fuel to the combustion chamber through at least two separately controllable fuel supply systems or stages including separate fuel nozzles or sets of fuel nozzles. As the power level of the machine is increased, the amount of fuel supplied through respective stages is increased to achieve a desired power level. Multiple fuel stages provide flexibility in the operation of a gas turbine engine, since the relative fuel flow rate through the various stages may be controlled to achieve a desired set of combustion conditions in the combustion chamber. For example, a Model 501F gas turbine engine supplied by the assignee of the present invention includes two main fuel premix stages, a diffusion pilot stage, and a C stage that premixes a small amount of fuel into the compressed air upstream of all of the other stages. Such a multi-stage system provides a high degree of control to the plant operator for ensuring stable operation with minimized emissions under a broad range of operating conditions.
FIG. 1 illustrates a portion of a prior art can annular combustor 10 of the general type described in U.S. Pat. No. 6,082,111. A combustor basket 12 contains openings for receiving a flow of combustion air 14 from a compressor 16. The air 14 is directed through either a diffusion pilot burner assembly 18 or one of eight premix main fuel burner assemblies 20 (also called rockets) disposed in a ring around the diffusion pilot burner 18. Alternate ones of the main fuel burner assemblies 20 may be grouped into separate stages, for example an A-stage that is provided with a combustible fuel through A-stage fuel supply line 22 and a B-stage that is provided with a separately controllable combustible fuel through B-stage fuel supply line 24. A diffusion pilot fuel supply 26 provides a flow of a combustible fuel to diffusion pilot nozzle 28 having an outlet 30 for delivering diffusion pilot fuel 32 to combustion chamber 34. The pilot diffusion flame provides stability for the combustion of the mixture 36 of main fuel and air that is provided to the combustion chamber 34 by the premix main burner assemblies 20.
Increasingly strict environmental regulations continue to reduce the acceptable level of emissions produced by a gas turbine power plant. For example, while many in-service engines were originally designed to produce no more than about 25 ppm of nitrous oxides, new regulations in many regions now limit the production of NOx to less than 15 ppm or even less than 9 ppm. Thus continued improvements in gas turbine combustor design are needed. Furthermore, an economical manner of reducing the emissions from an existing fleet of gas turbine engines is needed.